Functionally graded coating

ABSTRACT

In one aspect, composite articles are described comprising multifunctional coatings. A composite article described herein, in some embodiments, comprises a substrate and a coating adhered to the substrate, the coating comprising an inner layer and an outer layer, the inner layer comprising a presintered metal or alloy and the outer layer comprising particles disposed in a metal or alloy matrix.

RELATED APPLICATION DATA

Pursuant to 35 U.S.C. §120, the present application is a continuation ofU.S. patent application Ser. No. 13/305,448 filed Nov. 28, 2011.

FIELD

The present invention relates to coatings and, in particular, tomultifunctional coatings for metallic substrates.

BACKGROUND

Coatings are often applied to equipment subjected to harsh environmentsor operating conditions in efforts to extend the useful lifetime of theequipment. Various coating identities and constructions are availabledepending on the mode of failure to be inhibited. For example, wearresistant, erosion resistant and corrosion resistant coatings have beendeveloped for metallic substrates.

A significant problem encountered in coating applications is prematurefailure or degradation of the coating. Coatings of metallic substratescan fail according to a variety of mechanisms, including delaminationand cracking/fracture. In some cases, a coated metal substrate issubjected to thermal cycling that can impair the bonding of the coatingto the substrate. In some applications, for example, the metal substrateis subjected to a post-coat heat treatment such as martermpering ornormalizing in order to improve the mechanical properties of thesubstrate, wherein the post-coat heat treatment fractures the coating.Moreover, in some cases, cracks in the coating can propagate into thesubstrate leading to additional problems. FIG. 1 illustrates fracture ofan abrasion resistant prior art coating bonded to a metal substrate, thefracture resulting from coating processes and/or heat treatment of thesubstrate. As illustrated in FIG. 1, the crack traversed the abrasionresistant coating.

Attempts have been made to provide coating architectures that areresistant to premature failure resulting from heat treatment, claddingprocesses and other environmental factors. In U.S. Pat. No. 5,352,526,for example, a composite coating is provided having a soft metalliclayer under a hardface coating, the soft metallic layer having a crackarrest functionality. The composite coating of U.S. Pat. No. 5,352,526is formed by stacking preform layers on the metal substrate surface andheating the preform layers in a single step to provide the compositecoating. A preform comprising particles of the soft metallic underlayeris applied to the substrate followed by application of a preform ofrefractory particles, such as tungsten carbide. A braze filler preformis applied to the refractory particle preform and all three preforms areheated simultaneously to provide the coating. The braze filler materialtop layer is infused by capillary action into both the porous refractoryparticle layer and the porous soft metal particle layer yielding anessentially void free coating. While demonstrating sufficient crackarrest properties, the soft layer permitted crack propagation beyond theinterface with the hard particle layer, thereby compromising themechanical and corrosion resistant properties of the soft layer.

SUMMARY

In one aspect, composite articles are described herein comprisingmultifunctional coatings. In some embodiments, multifunctional coatingsof composite articles described herein are operable to increase theabrasion/wear resistance and corrosion resistance of the articles whileinhibiting coating failure modes, including delamination and/orfracture.

A composite article described herein, in some embodiments, comprises asubstrate and a coating adhered to the substrate, the coating comprisingan inner layer and an outer layer, the inner layer comprising a metal oralloy layer having porosity less than 40% by volume and the outer layercomprising particles disposed in a metal or alloy matrix.

In another embodiment, a composite article described herein comprises asubstrate and a coating adhered to the substrate, the coating comprisingan inner layer and an outer layer, the inner layer comprising apresintered metal or alloy and the outer layer comprising particlesdisposed in a metal or alloy matrix. In some embodiments, thepresintered metal or alloy inner layer is fully dense or substantiallyfully dense. Alternatively, in some embodiments, the presintered metalor alloy inner layer has porosity penetrated by the metal or alloymatrix of the outer layer.

In another embodiment, a composite article described herein comprises asubstrate and a coating adhered to the substrate, the coating comprisingan inner layer, an outer layer and an interfacial transition regionbetween the inner layer and the outer layer, wherein the inner layercomprises a substantially fully dense metal or alloy and the outer layercomprises particles disposed in a metal or alloy matrix. In someembodiments, the interfacial transition region demonstrates a structuredifferent from the inner layer and the outer layer.

In some embodiments of composite articles described herein, thesubstrate comprises a metal or alloy. Moreover, in some embodiments,coatings described herein are metallurgically bonded to the metal oralloy substrate.

In another aspect, methods of making composite articles are describedherein. In some embodiments, a method of making a composite articlecomprises disposing over a surface of a substrate a sheet comprising apowder metal or powder alloy composition, heating the sheet to providean inner layer comprising a sintered metal or sintered alloy adhered tothe substrate. A particulate composition comprising hard particles in acarrier is disposed over the sintered metal or sintered alloy of theinner layer and a brazing alloy composition is disposed over theparticulate composition. The particulate composition and the brazingalloy composition are heated to provide an outer layer comprising thehard particles disposed in an alloy matrix. In some embodiments, theouter layer is metallurgically adhered to the inner layer.

In some embodiments, the sheet comprising the powder metal or powderalloy composition is heated under conditions sufficient to provide afully dense or substantially fully dense sintered metal or sinteredalloy inner layer. Alternatively, the sheet comprising the powder metalor powder alloy composition, in some embodiments, is heated underconditions to yield a sintered metal or sintered alloy inner layerhaving porosity. In some embodiments wherein the sintered metal orsintered alloy inner layer displays porosity, the porosity is permeatedwith the alloy matrix of the outer layer to provide a fully dense orsubstantially fully dense inner layer.

Moreover, in some embodiments of methods described herein, the powdermetal or powder alloy composition is disposed in a liquid carrier asopposed to a sheet and applied to a surface of the substrate. The powdermetal or powder alloy composition is subsequently heated to provide aninner layer comprising the sintered metal or sintered alloy adhered tothe substrate. The sintered metal or sintered alloy can be fully denseor can demonstrate porosity, depending on heating conditions. Theparticulate composition and brazing alloy composition can besubsequently applied over the sintered metal or sintered alloy innerlayer and heated to provide an outer layer comprising the hard particlesdisposed in an alloy matrix.

Coatings of composite articles and methods described herein, in someembodiments, are multifunctional. In some embodiments, for example, ametal or alloy inner layer displays crack arrest and/or corrosionresistant functionalities. Additionally, the coating outer layercomprising particles disposed in a metal or alloy matrix, in someembodiments, is resistant to abrasion and/or erosion.

These and other embodiments are described in greater detail in thedetailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art abrasion resistant coating applied to ametal substrate, wherein the abrasion resistant coating has fracturedfrom local stresses induced during coating processes and/or post-coatheat treatment of the metal substrate.

FIG. 2 illustrates a coating comprising an interfacial transition regionaccording to one embodiment described herein.

FIG. 3 illustrates densification of a powder alloy of a coating innerlayer as a function of heating or sintering temperature according to oneembodiment described herein.

FIG. 4 is a cross-section metallography of a sintered continuous alloyof a coating inner layer metallurgically bonded to a substrate accordingto one embodiment described herein.

FIG. 5 is a cross-section metallography of a composite article accordingto one embodiment described herein.

FIG. 6 is a cross-section metallography of a composite article accordingto one embodiment described herein.

FIG. 7 is a cross-section metallogrphy of a porous sintered alloy of acoating inner layer according to one embodiment described herein.

FIG. 8 is a cross-section metallography of the sintered alloy of FIG. 7after application of an abrasive outer layer to the sintered alloyaccording to one embodiment described herein.

FIG. 9 is a cross-section metallography of a sintered alloy of an innercorrosion resistant and crack arrest layer of a composite articleaccording to one embodiment described herein.

FIG. 10 is a cross-section metallography of a composite articleaccording to one embodiment described herein.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by referenceto the following detailed description and examples and their previousand following descriptions. Elements, apparatus and methods describedherein, however, are not limited to the specific embodiments presentedin the detailed description and examples. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations will bereadily apparent to those of skill in the art without departing from thespirit and scope of the invention.

In one aspect, composite articles are described herein comprisingmultifunctional coatings. In some embodiments, multifunctional coatingsof composite articles described herein are operable to increase theabrasion/wear resistance and corrosion resistance of the articles whileinhibiting coating failure modes, including delamination and/orfracture.

A composite article described herein, in some embodiments, comprises asubstrate and a coating adhered to the substrate, the coating comprisingan inner layer and an outer layer, the inner layer comprising a metal oralloy layer having porosity less than 40% by volume and the outer layercomprising particles disposed in a metal or alloy matrix. As describedfurther herein, the metal or alloy of the inner layer, in someembodiments, comprises a presintered metal or alloy. Alternatively, insome embodiments, the metal or alloy comprises weld overlay, plasmatransferred arc, thermal spray, cold spray or laser clad deposited metalor alloy. In some embodiments, for example, weld overlay includes rodweld overlay, wire weld overlay or powder weld overlay. In someembodiments, the metal or alloy of the inner layer is applied byinfrared cladding or induction cladding.

In another embodiment, a composite article described herein comprises asubstrate and a coating adhered to the substrate, the coating comprisingan inner layer and an outer layer, the inner layer comprising apresintered metal or alloy and the outer layer comprising particlesdisposed in a metal or alloy matrix.

Turning now to components of composite articles described herein, acomposite article described herein comprises a substrate. In someembodiments, a substrate comprises a metal or alloy. A substrate, forexample, can comprise iron alloys, nickel alloys, cobalt alloys or otheralloys. In some embodiments, substrates comprise cast iron, low-carbonsteels, alloy steels, tool steels or stainless steels. In someembodiments, a substrate comprises a refractory material. Moreover,substrates can comprise various geometries. In some embodiments, asubstrate has a cylindrical geometry, wherein the inner diameter (ID)surface, outer diameter (OD) surface or both are coated with a coatingdescribed herein. In some embodiments, for example, substrates comprisebearings, extruder barrels, extruder screws, flow control components,roller cone bits, fixed cutter bits, piping or tubes. In someembodiments, piping comprises boiler piping or piping/tubes subject toharsh environmental conditions, including high erosion conditions.

A composite article described herein comprises a coating adhered to thesubstrate, the coating, in some embodiments, having an inner layercomprising a presintered metal or alloy. In such embodiments, the metalor alloy of the inner layer is termed “presintered” since metal or alloyparticles are sintered to provide the inner layer of the coating priorto application or formation of the outer layer of the coating. Sinteringmetal or alloy particles to provide a sintered metal or alloy innerlayer adhered to the substrate prior to application of the coating outerlayer is a fundamental structural departure from prior coatings whereinmetal or alloy particles are infiltrated and/or encapsulated with abraze alloy in the simultaneous production of the inner and outer layersof the coating.

Suitable metals or alloys for the coating inner layer can be selectedaccording to various considerations including, but not limited to, thecompositional identity of the substrate, desired hardness of the innerlayer and/or the desired compositional identity of the metal or alloymatrix of the outer layer.

In some embodiments, the inner layer comprises presintered nickel. Theinner layer, in some embodiments, comprises a presintered nickel-basedalloy. Nickel-based alloys for use in some embodiments of a coatinginner layer contain additive elements of varying contents. Additiveelements can include boron, aluminum, carbon, silicon, phosphorous,titanium, zirconium, yttrium, rare earth elements, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt,copper or silver or combinations thereof. In some embodiments,presintered nickel-based alloys suitable for an inner layer havecompositional parameters derived from Table I:

TABLE I Presintered Ni-Alloy Composition Element Amount (wt. %) Cobalt0-15 Chromium 1-30 Molybdenum 2-28 Tungsten 0-5  Iron 0-50 Niobium 0-6 Silicon 0-1  Manganese 0-2  Copper 0-3  Aluminum 0-1  Titanium 0-2 Nickel BalanceIn some embodiments, a presintered nickel-based alloy of an inner layercomprises a nickel-iron alloy, such as Ni-30Fe or a nickel-chromiumalloy, such as Ni-20Cr or Ni-10Cr. Additionally, in some embodiments, apresintered nickel-based alloy comprises a nickel-copper alloy such asNi-55Cu or Ni-30Cu. In one embodiment, a nickel-based alloy comprisesNi-2Mn-2Al-1Si. Nickel-based alloys, in some embodiments, arecommercially available under the HASTELLOY®, INCONEL® and/or BALCO®trade designations.

A presintered alloy of an inner layer, in some embodiments, comprisescopper-based alloys. Additive elements for copper-based alloys caninclude beryllium, aluminum, nickel, chromium, cobalt, manganese, iron,silicon, zinc, zirconium, lead, tungsten, titanium, tantalum, niobium,boron or phosphorous or combinations thereof. In some embodiments, apresintered copper-based alloy of an inner layer comprises Cu-45Ni,Cu-10Ni, Cu-(18-27)Ni-(18-27)Mn or Cu-(29-32)Ni-(1.7-2.3)Fe-(1.5-2.5)Mn.

In some embodiments, an inner layer comprises presintered cobalt or apresintered cobalt-based alloy. Additive elements for cobalt-basedalloys can comprise chromium, molybdenum, tungsten, nickel, iron, boron,carbon, nitrogen, phosphorous, aluminum, silicon, manganese, titanium,vanadium, niobium, tantalum, zirconium, yttrium or copper orcombinations thereof. In some embodiments, cobalt alloys arecommercially available under the trade designation STELLITE® and/orMEGALLIUM®.

Moreover, in some embodiments, the inner layer comprises presinteredstainless steel. In some embodiments, stainless steels of the innerlayer comprise austenic stainless steels, including 300 series stainlesssteels (e.g. 304, 316, 317, 321, 347) and 600 series stainless steels(e.g., 630-635, 650-653, 660-665). In some embodiments, stainless steelsof the inner layer comprise ferritic stainless steels, such as thosecontaining 10-27% chromium with marginal nickel contents. Stainlesssteels of the inner layer, in some embodiments, comprise duplexstainless steels or specialty iron-based alloys, includingFe-24Ni-20.5Cr-6.2Mo,Fe—Ni(32.5-35)-Cr(19-21)-Cu(3-4)-Mo(2-3)-Mn(<2)-Si(<1).

As described herein, the presintered metal or alloy of the coating innerlayer, in some embodiments, is fully dense or substantially fully denseprior to application and/or formation of the outer layer of the coating.Alternatively, in some embodiments, the presintered metal or alloy ofthe inner layer has porosity. The porosity of the presintered metal oralloy, in some embodiments, is less than about 40% by volume. In someembodiments, the porosity of the presintered metal or alloy of the innerlayer is less than about 30% by volume. In some embodiments, theporosity of the presintered metal or alloy of the inner layer is lessthan about 20% by volume. The porosity of the presintered metal oralloy, in some embodiments, is less than about 10% by volume. In someembodiments, the porosity of the presintered metal or alloy of the innerlayer is less than about 5% by volume. In some embodiments, the porosityof the presintered metal or alloy of the inner layer is substantiallyuniform. In some embodiments, the porosity of the presintered metal oralloy of the inner layer is interconnected.

As discussed further herein, the porosity of a presintered metal oralloy of the inner layer, in some embodiments, is penetrated by thebrazing metal or alloy matrix of the outer layer. In some embodiments,the porosity of the presintered metal or alloy of the inner layer ispermeated or infiltrated with the brazing metal or alloy matrix of theouter layer to provide a fully dense or substantially fully dense innerlayer.

In some embodiments of a composite article described herein, the innerlayer of the coating further comprises particles disposed in thepresintered metal or alloy. In such embodiments, the presintered metalor alloy acts as a matrix for the particles. Particles suitable for usewith the metal or alloy matrix of the inner layer can comprise hardparticles including, but not limited to, particles of metal carbides,metal nitrides, metal borides, metal silicides, ceramics, cementedcarbides or cast carbides or mixtures thereof. Hard particles describedherein can comprise precipitates and/or additive particles.

In some embodiments, hard particles comprise carbides of tungsten,titanium, chromium, molybdenum, zirconium, hafnium, tanatalum, niobium,rhenium, vanadium, iron, boron or silicon or mixtures thereof. Hardparticles, in some embodiments, comprise nitrides of aluminum, boron,silicon, titanium, zirconium, hafnium, tantalum or niobium or mixturesthereof. Additionally, in some embodiments, hard particles compriseborides such as titanium di-boride and tantalum borides or silicidessuch as MoSi₂. Hard particles, in some embodiments, comprise crushedcemented carbide, crushed carbide, crushed nitride, crushed boride orcrushed silicide or combinations thereof. In some embodiments, hardparticles comprise intermetallic compounds such as nickel aluminide.

Hard particles can be present in the presintered metal or alloy matrixof a coating inner layer in any amount not inconsistent with theobjectives of the present invention. In some embodiments, hard particlesare present in the metal or alloy matrix in an amount less than about 20volume percent. In some embodiments, hard particles are present in themetal or alloy matrix in an amount less than about 10 volume percent.Hard particles, in some embodiments, are present in an amount less thanabout 5 volume percent.

The inner layer of the coating having a construction described hereincan have any thickness not inconsistent with the objectives of thepresent invention. In some embodiments, the inner layer has a thicknessof at least about 100 μm. In some embodiments, the inner layer has athickness ranging from about 200 μm to about 5 mm. The inner layer, insome embodiments, has a thickness ranging from about 500 μm to about 2mm. In some embodiments, the inner layer has a thickness ranging fromabout 500 μm to about 1 mm. In some embodiments, the inner layer has athickness ranging from about 200 μm to about 1 mm. In some embodiments,the inner layer has a thickness ranging from about 300 μm to about 800μm.

In some embodiments, the inner layer of the coating having aconstruction described herein is metallurgically bonded to thesubstrate. Moreover, in some embodiments, the inner layer of the coatinghaving a construction described herein has a hardness according to theRockwell C scale (HRC) of less than about 40. In some embodiments, theinner layer of the coating has a hardness less than about 36 HRC. Insome embodiments, the inner layer of the coating has a hardness lessthan about 30 HRC. In embodiments, wherein hard particles are present inthe presintered metal or alloy, the foregoing HRC values are determinedfrom the metal or alloy. HRC values recited herein are determinedaccording to ASTM E18-08b Standard Test Method for Rockwell Hardness ofMetallic Materials.

The inner layer of the coating, in some embodiments, has a substantiallyuniform finish prior to application or deposition of the outer layer. Insome embodiments, for example, the inner layer has a surface roughness(Ra_(μinches)) less than about 250 prior to deposition of the outerlayer. In some embodiments, the inner layer has a surface roughness ofless than about 200 Ra or less than about 100 Ra prior to deposition ofthe outer layer. The inner layer, in some embodiments, has a surfaceroughness ranging from about 20 Ra to about 250 Ra or from about 30 Rato about 125 Ra prior to deposition of the outer layer. In someembodiments, the inner layer is provided the desired surface roughnessprior to deposition of the outer layer by mechanical means such asgrinding, sand/grit blasting or combinations thereof. Surface roughnessvalues recited herein are determined according to ASTM D7125-05 StandardTest Method for Measurement of Surface Roughness of Abrasive BlastCleaned Metal Surfaces Using a Portable Stylus Instrument.

The inner layer of a coating described herein, in some embodiments,functions as a crack arrest layer. In some embodiments, the inner layerfunctions as a corrosion resistant layer. Moreover, in some embodiments,the inner layer functions as a crack arrest layer and a corrosionresistant layer.

A coating of a composite article described herein also comprises anouter layer. In some embodiments, the coating outer layer functions asan abrasion resistant and/or erosion resistant layer. The outer layer ofa coating described herein comprises particles disposed in a metal oralloy matrix. The metal or alloy matrix of the outer layer can beselected according various considerations including, but not limited to,the compositional identity of the metal or alloy of the inner layer, thecompositional identity of the substrate and/or the compositionalidentity of the particles to be disposed in the metal or alloy matrix ofthe outer layer. In some embodiments, for example, the metal or alloymatrix of the outer layer has a melting point or solidus temperaturelower than the metal or alloy inner layer, the substrate and/or theparticles disposed in the metal or alloy matrix. The melting point ofthe metal or alloy matrix of the outer layer, in some embodiments, is atleast 100° C. lower than the melting point of the metal or alloy innerlayer. In some embodiments, the melting point of the metal or alloymatrix of the outer layer is at least 200° C. lower than the meltingpoint of the metal or alloy inner layer.

In some embodiments, the metal or alloy matrix of the outer layer is abrazing metal or brazing alloy. Any brazing metal or alloy notinconsistent with the objectives of the present invention can be used asthe matrix of the outer layer. In some embodiments, for example, thealloy matrix of the outer layer comprises nickel-based alloys havingcompositional parameters derived from Table II:

TABLE II Outer Layer Ni-Based Alloy Matrix Compositional ParametersElement Amount (wt. %) Chromium 3-28 Boron 0-6  Silicon 0-15 Phosphorous0-12 Iron 0-6  Carbon 0-1  Copper 0-50 Molybdenum 0-5  Niobium 0-5 Tantalum 0-5  Tungsten 0-20 Nickel BalanceIn some embodiments, the alloy matrix of the outer layer is selectedfrom the Ni-based alloys of Table III.

TABLE III Outer Layer Ni-Based Alloy Matrix Compositional ParametersNi-Based Alloy Compositional Parameters (wt. %) 1 Ni—15%Cr—3%B—0.06%C 2Ni—14%Cr—4.5%Si—4.5%Fe—3.0%B—C 3 Ni—4.5%Si—3.5%B—C 4 Ni—14%Cr—10%P—C 5Ni—25%Cr—10%P 6 Ni—19%Cr—10.2%Si—C 7 Ni—22%Cr—6.5%Si—4.5%P 8Ni—15%Cr—8%Si 9 Ni—17%Cr—9%Si—0.1%B 10 Ni—7.0%Cr—5.0%P—50%Cu 11Ni—4.9%Cr—65%Cu—3.5%P 12 Ni—(13-15)%Cr—(2.75-3.5)%B—(4.5-5.0)%Si—(4.5-5.0)%Fe—(0.6-0.9)%C 13 Ni—(18.6-19.5)%Cr—(9.7-10.5)%Si

The alloy matrix of the outer layer, in some embodiments, comprisescopper-based alloys. Suitable copper-based alloys can comprise additiveelements of nickel (0-50%), manganese (0-30%), zinc (0-45%), aluminum(0-10%), silicon (0-5%), iron (0-5%) as well as other elements includingphosphorous, chromium, beryllium, titanium and/or lead. In someembodiments, the alloy matrix of the outer layer is selected from theCu-based alloys of Table IV.

TABLE IV Outer Layer Cu-Based Alloy Matrix Compositional ParametersCu-Based Alloy Compositional Parameters 1 Cu—25%Ni—25%Mn 2Cu—20%Ni—20%Mn 3 Cu—10%Ni 4 Cu—(29-32)% Ni—(1.7-2.3)%Fe—(1.5-2.5)% Mn 5Cu—(2.8-4.0)%Si—1.5%Mn— 1.0%Zn—1.0%Sn—Fe—Pb 6Cu—(7.0-8.5)Al—(11-14)%Mn—2- 4)%Fe—(1.5-3.0)%Ni

The alloy matrix of the outer layer, in some embodiments, comprisescobalt-based alloys. Suitable cobalt-based alloys can comprise additiveelements of chromium, nickel, boron, silicon, tungsten, carbon,phosphorous as well as other elements. In one embodiment, a cobalt-basedalloy of the outer layer has the compositional parameters of Co-17%Ni-19% Cr-4.0% W-8.0% Si-0.8% B-0.4% C.

As described herein, the outer layer of the coating comprises particlesdisposed in the metal or alloy matrix. Particles suitable for use in themetal or alloy matrix of the outer layer can comprise hard particles.Hard particles of the outer layer, in some embodiments, compriseparticles of metal carbides, metal nitrides, metal carbonitrides, metalborides, metal silicides, cemented carbides, cast carbides or otherceramics or mixtures thereof. In some embodiments, metallic elements ofhard particles of the outer layer comprise aluminum, boron and/or one ormore metallic elements selected from the group consisting of metallicelements of Groups IVB, VB, and VIB of the Periodic Table. Groups of thePeriodic Table described herein are identified according to the CASdesignation. Hard particles, in some embodiments, comprise tungstencarbide, boron nitride or titanium nitride or mixtures thereof.

Hard particles of the outer layer can have any size not inconsistentwith the objectives of the present invention. In some embodiments, hardparticles of the outer layer have a size distribution ranging from about0.1 μm to about 1 mm. Hard particles, in some embodiments, have a sizedistribution ranging from about 1 μm to about 500 μm. In someembodiments, hard particles have a size distribution ranging from about10 μm to about 300 μm. In some embodiments, hard particles have a sizedistribution ranging from about 50 μm to about 150 μm. In someembodiments, hard particles have a size distribution ranging from 10 μmto 50 μm. Hard particles, in some embodiments, demonstrate bimodal ormulti-modal size distributions.

Hard particles of the outer layer can have any desired shape orgeometry. In some embodiments, hard particles have spherical orelliptical geometry. In some embodiments, hard particles have apolygonal geometry. In some embodiments, hard particles have irregularshapes, including shapes with sharp edges.

Hard particles can be present in the metal or alloy matrix of the outerlayer in any amount not inconsistent with the objectives of the presentinvention. Hard particle loading of the outer layer can be variedaccording to several considerations including, but not limited to, thedesired hardness, abrasion resistance and/or toughness of the outerlayer. In some embodiments, hard particles are present in the metal oralloy matrix of the outer layer in an amount ranging from about 20volume percent to about 90 volume percent. Hard particles, in someembodiments, are present in the outer layer in an amount ranging fromabout 30 volume percent to about 85 volume percent. In some embodiments,hard particles are present in the outer layer in an amount ranging fromabout 40 volume percent to about 70 volume percent.

In some embodiments, the outer layer can further comprise abrasionresistant ceramic tiles, metal matrix composite tiles, crushed cementedcarbides or mixtures thereof in the metal or alloy matrix of the outerlayer for increasing the abrasion resistance of the outer layer. Ceramicand/or metal matrix composite tiles and crushed cemented carbides, insome embodiments, have a size ranging from about 1 mm to about 50 mm inat least one dimension.

The outer layer of the coating can have any thickness not inconsistentwith the objectives of the present invention. In some embodiments, outerlayer thickness is selected according to several considerations, such asthe desired abrasion/erosion characteristics and/or lifetime of theouter layer. In some embodiments, the outer layer has a thickness of atleast about 100 μm or at least about 500 μm. In some embodiments, theouter layer has a thickness of at least about 750 μm or at least about 1mm. The outer layer, in some embodiments, has a thickness ranging fromabout 100 μm to about 5 mm. In some embodiments, the outer layer has athickness ranging from about 500 μm to about 2 mm.

The outer layer of the coating comprising particles disposed in a metalor alloy matrix, in some embodiments, is metallurgically bonded to themetal or alloy inner layer. As described herein, a presintered metal oralloy of the inner layer, in some embodiments, is fully dense orsubstantially fully dense prior to application or formation of the outerlayer. In some embodiments wherein the presintered metal or alloy of theinner layer is fully dense or substantially fully dense, fabrication ofthe outer layer over the presintered inner layer produces an interfacialtransition region between the outer layer and the inner layer. In someembodiments, for example, the braze metal or alloy matrix of the outerlayer diffuses into a surface region of the fully dense presinteredmetal or alloy of the inner layer to establish the interfacialtransition region. The interfacial transition region, in someembodiments, has a structure different from the outer layer anddifferent from the inner layer. Additionally, the interfacial transitionregion, in some embodiments, has a thickness ranging from about 1 μm toabout 200 μm or from about 5 μm to about 100 In some embodiments, theinterfacial transition region has a thickness ranging from about 10 μmto about 50 μm.

FIG. 2 illustrates a coating comprising an interfacial transition regionaccording to one embodiment described herein. The coating of FIG. 2comprises a corrosion and crack resistant inner layer comprising a fullydense presintered alloy and an abrasion resistant outer layer comprisinghard particles disposed in a metal or alloy matrix metallurgicallybonded to the inner layer. An interfacial transition region ispositioned between the abrasion resistant outer layer and thecorrosion/crack resistant inner layer, the interfacial transition regionhaving a structure different from the outer layer and the inner layer.

Alternatively, in some embodiments, a presintered metal or alloy of theinner layer has porosity prior to application or formation of the outerlayer. The porosity of the presintered metal or alloy, in someembodiments, is penetrated by the metal or alloy matrix of the outerlayer during formation or construction of the outer layer over the innerlayer. In some embodiments, for example, the metal or alloy matrix ofthe outer layer permeates or infiltrates the porosity of the presinteredmetal or alloy to provide a fully dense or substantially fully denseinner layer.

The outer layer of a coating having a construction described herein, insome embodiments, has a hardness (HRC) greater than the inner layer ofthe coating. In some embodiments, the outer layer has a hardness of atleast about 30 HRC. In some embodiments, the outer layer has a hardnessof at least about 35 HRC. The outer layer, in some embodiments, has ahardness of at least about 40 HRC. In some embodiments, the outer layerhas a hardness of at least about 45 HRC. In some embodiments, the outerlayer has a hardness ranging from about 40 HRC to about 75 HRC.

Moreover, the outer layer of a coating having a construction describedherein, in some embodiments, has an abrasion resistance greater than theinner layer and/or the substrate. Abrasion resistance recited herein isdetermined based on adjusted volume loss measured in accordance withProcedure A of ASTM G65 Standard Test Method for Measuring AbrasionUsing the Dry Sand/Rubber Wheel. In some embodiments, the outer layerhas an adjusted volume loss less than 0.02 cm³ or less than about 0.012cm³. The outer layer, in some embodiments, has an adjusted volume lossless than 0.01 cm³ or less than about 0.008 cm³.

In some embodiments, a composite article described herein furthercomprises one or more layers of refractory material deposited over theouter layer by chemical vapor deposition (CVD), physical vapordeposition (PVD) or combinations thereof. CVD and/or PVD layer(s)deposited over the outer layer, in some embodiments, comprise ceramics,diamond, diamond-like carbon, tungsten carbide or combinations thereof.In some embodiments, the CVD and/or PVD layer(s) deposited over theouter layer comprise aluminum and/or one or more metallic elementsselected from the group consisting of metallic elements of Groups IVB,VB and VIB of the Periodic Table and one or more non-metallic elementsselected form the group consisting of non-metallic elements of GroupsIIIA, IVA, and VIA of the Periodic Table. In some embodiments, therefractory layer(s) are deposited over the outer layer by lowtemperature or medium temperature CVD.

In another aspect, a composite article described herein comprises asubstrate and a coating adhered to the substrate, the coating comprisingan inner layer, an outer layer and an interfacial transition regionbetween the inner layer and the outer layer, the inner layer comprisinga fully dense or substantially fully dense metal or alloy, and the outerlayer comprising particles disposed in a metal or alloy matrix.

In some embodiments, the fully dense or substantially fully dense metalor alloy of the inner layer displays a structure or constructionconsistent with being deposited by one of weld overlay, plasmatransferred arc, thermal spray, cold spray, laser cladding, infraredcladding, induction cladding or other cladding technologies. Depositionof the metal or alloy of the inner layer by weld overlay, plasmatransferred arc, thermal spray, cold spray, laser cladding, infraredcladding, induction cladding or other cladding technologies provides theinner layer a structure divergent from the foregoing embodiments whereinthe metal or alloy of the inner layer is presintered. The outer layer ofthe coating comprising particles disposed in a metal alloy or matrix,however, can have any construction consistent with that recitedhereinabove for the outer layer.

The interfacial transition region between the inner layer and the outerlayer, in some embodiments, has a structure different from the innerlayer and different from the outer layer. Additionally, the interfacialtransition region, in some embodiments, has a thickness ranging fromabout 1 μm to about 150 μm. In some embodiments, the interfacialtransition region between the inner layer and the outer layer has athickness ranging from about 5 μm to about 100 μm. In some embodiments,the interfacial transition region has a thickness ranging from about 10μm to about 50 μm.

In another aspect, methods of making a composite article are describedherein. In some embodiments, a method of making a composite articlecomprises disposing over a surface of a substrate a sheet comprising apowder metal or powder alloy composition, heating the sheet to providean inner layer comprising a sintered metal or sintered alloy adhered tothe substrate. A particulate composition comprising hard particles in acarrier is disposed over the sintered metal or sintered alloy of theinner layer and a brazing alloy composition is disposed over theparticulate composition. The particulate composition and the brazingalloy composition are heated to provide an outer layer comprising thehard particles disposed in an alloy matrix. In some embodiments, theouter layer is adhered to the inner layer.

Turning now to steps of methods described herein, a method describedherein comprises disposing over the surface of a substrate a sheetcomprising a powder metal or powder alloy composition. In someembodiments, the sheet comprising the powder metal or powder alloycomposition is cloth-like in nature. The sheet, in some embodiments,comprises an organic material. In some embodiments, the sheet comprisesone or more polymeric materials. Suitable polymeric materials for use inthe sheet, in some embodiments, comprise one or more fluoropolymersincluding, but not limited to, polytetrafluoroethylene (PTFE).

In some embodiments, the desired powder metal or powder alloycomposition of the coating inner layer of the composite article isselected and combined with an organic material, such as a polymericpowder, for the formation of the sheet. Any metal or alloy compositionrecited herein for the inner layer can be combined or blended with anorganic material for the formation of the sheet. In some embodiments,for example, a powder alloy having compositional parameters selectedfrom Table I herein is combined with an organic material. The organicmaterial and the powder metal or powder alloy composition aremechanically worked or processed to trap the metal or alloy powder inthe organic material. In one embodiment, for example, the desiredpowdered metal or powder alloy composition is mixed with 3-10% PTFE involume and mechanically worked to fibrillate the PTFE and trap thepowder metal or powder alloy. Mechanical working can include rolling,ball milling, stretching, elongating, spreading or combinations thereof.In some embodiments, the sheet comprising the powder metal or powderalloy is subjected to cold isostatic pressing. In some embodiments, theresulting sheet comprising the powder metal or powder alloy has a lowelastic modulus and high green strength. In some embodiments, a sheetcomprising a powder metal or powder alloy composition of the inner layeris produced in accordance with the disclosure of one or more of U.S.Pat. Nos. 3,743,556, 3,864,124, 3,916,506, 4,194,040 and 5,352,526, eachof which is incorporated herein by reference in its entirety.

Alternatively, the desired powder metal or powder alloy composition iscombined with a liquid carrier for application to the substrate. In someembodiments, for example, the powder metal or powder alloy is disposedin a liquid carrier to provide a slurry or paint for application to thesubstrate. Suitable liquid carriers for powder metal or powder alloycompositions described herein comprise several components includingdispersion agents, thickening agents, adhesion agents, surface tensionreduction agents and/or foam reduction agents. In some embodiments,suitable liquid carriers are aqueous based.

Powder metal or powder alloy compositions disposed in a liquid carriercan be applied to surfaces of the substrate by several techniquesincluding, but not limited to, spraying, brushing, flow coating, dippingand/or related techniques. The powder metal or powder alloy compositioncan be applied to the substrate surface in a single application ormultiple applications depending on desired thickness of the coatinginner layer.

Moreover, in some embodiments, powder metal or powder alloy compositionsdisposed in liquid carriers can be prepared and applied to substratesurfaces in accordance with the disclosure of U.S. Pat. No. 6,649,682which is hereby incorporated by reference in its entirety.

After being disposed over a surface of the substrate, the sheet orliquid carrier comprising the powder metal or powder alloy is heated toprovide the inner layer of the coating comprising the sintered metal orsintered alloy adhered to the substrate. The sheet or liquid carrier isdecomposed or burned off during the heating process. The sintered metalor sintered alloy resulting from the heating process can have anyproperty or combination of properties recited herein for a sinteredmetal or sintered alloy of the inner layer. In some embodiments, thesubstrate and sheet or liquid carrier comprising the powder metal orpowder alloy composition is heated in a vacuum, inert or reducingatmosphere at a temperature and for a time period where the integrity ofthe substrate is maintained and the powder metal or powder alloy isdensified to the desired amount. As known to one of skill in the art,heating conditions including temperatures, atmosphere and time aredependent on several considerations including the identity of thesubstrate, the identity of the powder metal or powder alloy and thedesired structure of the resulting sintered layer.

In some embodiments, the powder metal or powder alloy is heated underconditions sufficient to produce a fully dense or substantially fullydense sintered metal or sintered alloy inner layer. Alternatively, thepowder metal or powder alloy composition, in some embodiments, is heatedunder conditions sufficient to produce a sintered metal or sinteredalloy inner layer having a desired porosity. In some embodiments, forexample, the powder metal or powder alloy composition is heated underconditions to produce a sintered metal or sintered alloy having porosityas recited herein. In some embodiments, the powder metal or powder alloycomposition is subjected to hot isostatic pressing and/or othermechanical processing to achieve the desired densification.

FIG. 3 illustrates densification of a powder alloy of the inner layer asa function of heating or sintering temperature according to oneembodiment described herein. The powder alloy used to generate the curvein FIG. 3 had compositional parameters of 20-23% chromium, 8-10%molybdenum, up to 5% iron, 3.15-4.15% total of niobium and tantalum, andbalance nickel. Curves similar to the one illustrated in FIG. 3 can beused in the selection of heating or sintering conditions for powdermetal or powder alloy compositions to provide a sintered metal orsintered alloy inner layer having the desired densification.

In some embodiments, heating the substrate and powder metal or powderalloy composition metallurgically binds the resulting sintered metal orsintered alloy of the inner layer to the substrate. Additionally, insome embodiments, the substrate is cleaned prior to application of thesheet or liquid carrier comprising the powder metal or powder alloycomposition. Cleaning the substrate can be administered by chemicaltreatment, mechanical treatment or both. In some embodiments, forexample, a substrate is cleaned with sodium hydroxide solution and/orsubjected to grit or particle blasting.

An outer layer of the coating is subsequently applied over the innerlayer. A particulate composition in a carrier is subsequently disposedover the sintered metal or sintered alloy of the inner layer and abrazing alloy composition is disposed over the particulate composition.In some embodiments, the particulate composition comprises hardparticles. Suitable hard particles can comprise any of the hardparticles recited herein. In some embodiments, for example, hardparticles of the particulate composition comprise particles of metalcarbides, metal nitrides, metal borides, metal silicides, cementedcarbides, cast carbides or other ceramics or mixtures thereof.

A suitable carrier for the particulate composition, in some embodiments,comprises a sheet as described hereinabove for the powder metal orpowder alloy of the inner layer. In one embodiment, for example, anabrasive hard particulate composition comprising 94 volume percentcrushed cemented carbide particles, tungsten carbide particles ortitanium carbide particles or combinations thereof is mixed with 6% PTFEin volume, followed by mechanical working to provide the sheet.

Alternatively, in some embodiments, a suitable carrier for theparticulate composition comprising hard particles is a liquid carrier asdescribed herein for the powder metal or powder alloy of the innerlayer. In some embodiments, for example, hard particles disposed in aliquid carrier are applied to the sintered metal or sintered alloy ofthe inner layer by one or more of spraying, brushing, flow coating,dipping and/or related techniques. In some embodiments, hard particlesdisposed in a liquid carrier can be prepared and applied in accordancewith the disclosure of U.S. Pat. No. 6,649,682.

The brazing alloy composition disposed over the particulate compositionof hard particles, in some embodiments, comprises a brazing alloydescribed herein. In some embodiments, for example, the brazing alloycomposition is selected from Table II, Table III or Table IV herein. Insome embodiments, the brazing alloy composition is provided as a brazealloy powder disposed in a sheet as described hereinabove for the powdermetal or powder alloy of the coating inner layer. In some embodiments,the brazing alloy composition is provided as a thin sheet of the brazingalloy composition itself.

The particulate composition comprising hard particles and the brazingalloy composition are heated to provide an outer layer of the compositearticle comprising the hard particles disposed in a brazing alloymatrix. The outer layer resulting from the heating process can have anyproperty or combination of properties recited herein for an outer layer.As the brazing alloy composition has a melting point lower than themelting point of the sintered metal or sintered alloy of the innerlayer, the brazing alloy and particulate composition are heated to atemperature below the melting point of the sintered metal or sinteredalloy of the inner layer. In some embodiments, the brazing alloy andparticulate composition are heated to a temperature at least 100° C.below the melting point of the sintered metal or sintered alloy of theinner layer.

In some embodiments, the outer layer comprising hard particles disposedin the braze alloy matrix is metallurgically bonded to the sinteredmetal or sintered alloy of the inner layer. Moreover, in someembodiments wherein the sintered metal or sintered alloy of the innerlayer has porosity prior to application of the outer layer, the brazealloy of the outer layer penetrates the porosity. In some embodiments,for example, the braze alloy of the outer layer permeates or infiltratesporosity of the sintered metal or sintered alloy to provide a fullydense or substantially fully dense inner layer.

In some embodiments of methods described herein, the sintered metal orsintered alloy of the inner layer is processed to provide a desiredsurface roughness of the metal or alloy prior to application ordeposition of the outer layer. The sintered metal or alloy of the innerlayer, in some embodiments, is processed to provide a surface roughness(Ra_(μinches)) less than about 250 Ra. In some embodiments, the sinteredmetal or alloy of the inner layer is processed to provide a surfaceroughness less than about 200 Ra or less than about 100 Ra. The sinteredmetal or alloy of the inner layer, in some embodiments, is processed toprovide a surface roughness ranging from about 20 Ra to about 250 Ra orfrom about 30 Ra to about 125 Ra. The sintered metal or sintered alloyof the inner layer can be processed according to a variety of techniquesincluding mechanical means, such as grinding, sand/grit blasting orcombinations thereof.

In some embodiments of methods described herein, the metal or alloyinner layer is deposited by one of weld overlay, plasma transferred arc,thermal spray, cold spray, laser cladding, infrared cladding, inductioncladding or other cladding technologies. As further illustrated in theexamples to follow, deposition of the metal or alloy of the inner layerby weld overlay, plasma transferred arc, thermal spray, cold spray,laser cladding, infrared cladding, induction cladding or other claddingtechnologies provides the inner layer a structure divergent from theforegoing embodiments wherein the metal or alloy of the inner layer issintered.

Additionally, in some embodiments wherein the metal or alloy inner layeris deposited by weld overlay, plasma transferred arc, thermal spray,cold spray, laser cladding, infrared cladding, induction cladding orother cladding technologies, the outer layer is applied or depositedaccording to procedures set forth herein.

These and other embodiments are further illustrated by the followingnon-limiting examples.

EXAMPLE 1 Composite Article Comprising Functionally Graded Coating

A composite article having a construction described herein was producedas follows. Tungsten carbide powder (40% by volume 2 to 5 microns sizeparticles and 60% by volume-325 mesh size particles) was mixed with 6%by volume of PTFE. The mixture was mechanically worked to fibrillatePTFE and trap the tungsten carbide particles and then rolled, thusmaking a cloth-like flexible abrasive carbide sheet as fully describedin U.S. Pat. No. 4,194,040. A braze metal filler powder with compositionof 79-84% nickel, 13-19% chromium and 2-5% boron by weight was mixedwith 6% by volume of PTFE to form a cloth-like braze sheet, similar tothat of tungsten carbide sheet set forth above.

A corrosion and crack resistant alloy powder with a composition of20-23% chromium, 8-10% molybdenum, up to 5% iron, 3.15-4.15% total ofniobium and tantalum, and the balance nickel by weight was mixed with 6%by volume of PTFE to form a cloth-like corrosion and crack resistantalloy sheet, in the way similar to that of tungsten carbide sheet setforth above.

The cloth-like corrosion and crack resistant alloy sheet was applied tothe outer diameter (OD) surface of a 4140 steel tube substrate by meansof adhesive in preparation for the formation of an inner layer of thecoating. The sample was heated in a vacuum furnace to 1330° C., and heldat this temperature for approximately 60 minutes, during which the alloypowder was densified into an essentially porosity-free sinteredcontinuous alloy inner layer, metallurgically bonded to the substratesteel upon cooling. FIG. 4 is a cross-section metallography of thesintered continuous alloy inner layer illustrating the porosity-freenature of the sintered alloy and the metallurgical bonding of thesintered alloy inner layer to the substrate. After cooling, the surfaceof the sintered alloy was mechanically worked to a finish for applyingthe abrasive resistant layer.

The tungsten carbide sheet preform was applied on the corrosion andcrack resistant alloy surface of the inner layer by means of adhesiveand a braze filler sheet preform was glued in place over the tungstencarbide sheet preform. The sample was then heated in a vacuum furnace to1100° C.-1160° C. for approximately 15 minutes to 4 hours during whichthe braze preform melted and infiltrated the tungsten carbide preform,and upon cooling, a functionally graded coating/cladding was formedcomprising a tungsten carbide abrasive outer layer metallurgicallybonded to the corrosion and crack resistant sintered alloy inner layer.

The resulting composite article comprising the functionally gradedcoating/cladding was heat-treated by heating to 800° C.-950° C. in asalt bath, held for 1-4 hours, followed by quenching in a molten saltbath to 150° C.-250° C. The composite article was then tempered at 550°C.-750° C. in air for 3 hours. The hardness of steel substrate afterheat treatment was about 32 HRC. Visual examination of thecoating/cladding surface indicated a significant reduction in visiblecracks at the surface of the abrasive carbide outer layer compared withprior art coatings formed without a sintered alloy inner layer, such asthose demonstrated in one or more or U.S. Pat. Nos. 3,743,556,3,864,124, 3,916,506, 4,194,040, 5,236,116, 5,164,247, and 5,352,526.

Furthermore, metallographic examination of the cross section of thecoating/cladding of the present example, as illustrated in FIG. 5,indicated the absence of cracks penetrating into sintered alloycorrosion resistant/crack arresting inner layer. The cross sectionmetallography also indicated metallurgical bonding at the interfacesamong outer abrasive layer, inner corrosion/crack resistant layer, andsubstrate.

EXAMPLE 2 Composite Article Comprising Functionally Graded Coating

A composite article having a construction described herein was producedin accordance with the procedure of Example 1, the difference being thatthe corrosion and crack resistant alloy of the inner layer was heated toa temperature of 1300° C. and held for 60 minutes in the vacuum furnacefor densification and metallurgical bonding to the steel substratesurface. The resulting sintered alloy of the inner layer demonstratedporosity ranging from 3-7% in volume.

As in Example 1, the tungsten carbide sheet preform was then applied tothe sintered alloy surface by means of adhesive, and the braze fillersheet preform was glued in place over the tungsten carbide preform,followed by heating in a vacuum furnace to 1100° C.-1160° C. forapproximately 15 minutes to 4 hours. The braze preform thus melted andinfiltrated the inner alloy layer, as well as the carbide preform.Infiltration of the sintered alloy of the inner layer by the braze alloyreduced the porosity of the sintered alloy to provide a fully dense orsubstantially fully dense inner layer. As the result, the inner alloylayer was bonded metallurgically to both the outer abrasive carbidelayer and the underneath steel substrate. While displaying a differentmicrostructure from that formed in Example 1 (as shown in FIG. 4), thecontinuous, essentially porosity-free inner alloy layer was also crackresistant during a heat treatment administered as described inExample 1. FIG. 6 is a cross-section metallography of the compositearticle of the present example illustrating the absence of crackspenetrating into the sintered alloy corrosion resistant/crack arrestinginner layer after heat treatment of the composite article.

EXAMPLE 3 Composite Article Comprising Functionally Graded Coating

A composite article having a construction described herein was producedin accordance with the procedure of Example 1, the difference being thatthe corrosion and crack resistant alloy of the inner layer was heated toa temperature of 1200° C. and held for 60 minutes in the vacuum furnace.The resulting sintered alloy of the inner layer demonstrated a higherporosity ranging from 28-34% in volume. The sintered alloy inner layerbonded to the steel substrate as displayed in FIG. 7. FIG. 7additionally illustrates the porosity of the sintered alloy.

The tungsten carbide abrasive outer layer was deposited on the poroussintered alloy of the inner corrosion and crack resistant inner layer inaccordance with the process of Example 1. The braze alloy preform thusmelted and infiltrated the inner alloy layer, as well as the carbidepreform. Infiltration of the sintered alloy of the inner layer by thebraze alloy reduced the porosity of the sintered alloy to provide afully dense or substantially fully dense inner layer. As the result theinner alloy layer was bonded metallurgically to both the outer abrasivecarbide layer and the underneath steel substrate. FIG. 8 is across-section metallography of the composite article of the presentexample illustrating the abrasive carbide outer layer and the fullydense or substantially fully dense alloy of the corrosion/crackresistant inner layer produced by infiltration of the porous sinteredalloy by the braze of the outer layer.

The functionally graded coating of the present example was less crackresistant in comparison with Examples 1 and 2 during heat treatment.After a heat treatment was administered in accordance with Example 1,several cracks originated from the outer abrasive layer and wereterminated at the interface with the inner crack and corrosion resistantalloy layer. While demonstrating less crack resistance, the functionallygraded coating of the present example was an enhancement over priorcoatings.

EXAMPLE 4 Composite Article Comprising Functionally Graded Coating

A composite article having a construction described herein was producedin accordance with Example 1, the difference being that the innerdiameter (ID) of the steel substrate tube was coated. Cross sectionmetallography of the functionally graded coating/cladding on the IDafter heat treatment provided evidence of the same microstructure as thecoating on the OD in Example 1. Moreover, cracks did not penetrate intocorrosion/crack resistant inner layer. FIG. 9 displays a cross-sectionmetallography of the sintered alloy of the inner corrosion resistant andcrack arrest layer.

EXAMPLE 5 Composite Article Comprising Functionally Graded Coating

As described herein, a continuous corrosion and crack resistant innerlayer of a composite article may also be produced by laser cladding,plasma transferred arc, weld overlay and other techniques before addingan outer abrasive layer by clothing. In the present example, a corrosionand crack resistant layer of about 750 μm thick was deposited on a 4140steel substrate surface by plasma transferred arc using −325/+120 meshesof alloy powder of the same composition as that used in Example 1 (alloywire of the same composition may also be used). The surface of theresulting alloy layer was ground and sand-blasted, followed by adding anabrasive carbide outer layer in accordance with the process described inExample 1. FIG. 10 displays the cross section metallography after heattreating the composite article of the present example as described inExample 1. The inner alloy layer was fully dense or substantially fullydense and free of cracks.

Various embodiments of the invention have been described in fulfillmentof the various objects of the invention. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations thereof willbe readily apparent to those skilled in the art without departing fromthe spirit and scope of the invention.

That which is claimed is:
 1. A composite article comprising: asubstrate; and a coating adhered to the substrate, the coatingcomprising an inner layer and an outer layer, the inner layer beingsubstantially fully dense and comprising sintered metal or alloy, andthe outer layer comprising hard particles disposed in matrix metal ormatrix alloy, wherein the matrix metal or matrix alloy does notinfiltrate the inner layer.
 2. The composite article of claim 1, whereinthe inner layer is metallurgically bonded to the substrate.
 3. Thecomposite article of claim 1, wherein the hard particles of the outerlayer comprise one or more metal carbides, metal nitrides, metalborides, metal silicides, ceramics, cemented carbides, cast carbides ormixtures thereof.
 4. The composite article of claim 1, wherein the innerlayer has hardness on the Rockwell C scale lower than the outer layer.5. The composite article of claim 4, wherein the outer layer has anabrasion resistance greater than the inner layer as measured accordingto ASTM G65-04.
 6. The composite article of claim 1, wherein the outerlayer further comprises metal carbide tiles or ceramic tiles disposed inthe matrix metal or matrix alloy.
 7. The composite article of claim 1,wherein the inner layer further comprises particles disposed in thesintered metal or alloy, the particles selected from the groupconsisting of metal carbides, metal nitrides, metal borides, metalsilicides, ceramics, cemented carbides, cast carbides and mixturesthereof.
 8. The composite article of claim 1, wherein the coatingfurther comprises at least one layer of refractory material depositedover the outer layer by chemical vapor deposition or physical vapordeposition of a combination thereof.
 9. The composite article of claim1, wherein the inner layer of the coating has corrosion resistant andcrack arrest functionalities, and the outer layer of the coating has anabrasion resistant functionality, erosion resistant functionality orcombination thereof.
 10. The composite article of claim 1, wherein thesubstrate comprises steel.
 11. The composite article of claim 1, furthercomprising an interfacial transition region between the inner layer andthe outer layer.
 12. The composite article of claim 1, wherein the innerlayer comprises sintered nickel-based alloy.
 13. The composite articleof claim 12, wherein the nickel-based alloy is of composition 0-15 wt. %cobalt, 1-30 wt. % chromium, 2-28 wt. % molybdenum, 0-5 wt. % tungsten,0-50 wt. % iron, 0-6 wt. % niobium, 0-1 wt. % silicon, 0-2 wt. %manganese, 0-3 wt. % copper, 0-1 wt. % aluminum, 0-2 wt. % titanium andthe balance nickel.
 14. A composite article comprising: a substrate; anda coating adhered to the substrate, the coating comprising an innerlayer and an outer layer, the outer layer comprising hard particlesdisposed in matrix metal or matrix alloy, and the inner layer beingsubstantially fully dense and formed of a porous sintered metal orsintered alloy scaffold infiltrated with the matrix metal or matrixalloy of the outer layer.
 15. The composite article of claim 14, whereinthe inner layer is metallurgically bonded to the substrate.
 16. Thecomposite article of claim 14, wherein the particles of the outer layercomprise one or more metal carbides, metal nitrides, metal borides,metal silicides, ceramics, cemented carbides, cast carbides or mixturesthereof.
 17. The composite article of claim 14, wherein the inner layerhas hardness on the Rockwell C scale lower than the outer layer.
 18. Thecomposite article of claim 17, wherein the outer layer has an abrasionresistance greater than the inner layer as measured according to ASTMG65-04.
 19. The composite article of claim 14, wherein the substratecomprises steel.
 20. The composite article of claim 14, wherein theinner layer of the coating has corrosion resistant and crack arrestfunctionalities, and the outer layer of the coating has an abrasionresistant functionality, erosion resistant functionality or combinationthereof.